Ozone is known as a substance, in the nature, having an extremely strong oxidizing power and has been widening its applications over various industries. For instance, ozone is utilized in waterworks and sewage plants for the sterilizing and decolorizing treatments. Another advantage of ozone is the nature that it turns to harmless oxygen through autolysis with time lapse. Ozone, therefore, is appreciated as a chemical used for sterilizing and decolorizing processes which are easier and safer in handling than the former processes by chemicals, without secondary contamination by chemical residues or reaction by-products, offering an easy post-treatment.
Well known methods of ozone production include the UV lamp process, the silent discharge process, and the electrolysis process. The UV lamp process produces ozone through exciting oxygen by UV rays, available from a relatively simple unit, but the production volume is limited and therefore, popularly used for deodorizing rooms and cars. The silent discharge process is one of the most prevalent and commonly used ozone generation methods. It is widely applied for various purposes ranging from simple room deodorizing by a small-scale ozonizer to industrial water treatment by a large-scale ozonizer with an output capacity of several tens kilograms per hour. The silent discharge process uses oxygen gas or oxygen in air as feed material, and generates ozone through excitation by electric discharge.
Ozone generation processes that utilize water as feedstock are usually electrochemical processes that electrolyze water to produce oxidized end products, such as oxygen, ozone, and hydrogen. Water electrolysis occurs as described by the following equations:2H2O→O2+4H++4e− E0=1.229V  (1)3H2O→O3+6H++6e− E0=1.511V  (2)O2+H2O→O3+2H++2e− E0=2.075V  (3)
There are a variety of electrochemical processes that can accomplish these reactions and all have different production efficiencies.
In general, there are three basic cell designs: 1) flow cell composed of an anode and a cathode separated by a gap; 2) a cell composed of an anode and a cathode separated by a polymeric electrolyte, often a fluoropolymer-copolymer membrane that acts as a selective proton exchange membrane (PEM, proton exchange membrane); and 3) a cell that is composed of an anode and cathode separated and in direct contact with the polymeric membrane. By using a suitable anode material (usually Pt, PbO2, DSA, DLC, BDD), all cell designs can generate ozone to different extents. However, due to the extremely high oxidation potential of the reactions involved, the type of the anode dictates significantly the overall efficiency of production.
There are some anodes material, like Pt, on which reaction (1) is particularly thermodynamically and kinetically more favorable than reaction (2). This results in a, so-called, “solvent limit” that limits significantly the relative proportions of oxygen and ozone. In order to compensate for the lower ozone production, the voltage at the Pt electrode needs to be pushed higher, generating a significantly higher current due to a significant oxygen evolution. Furthermore, during the reaction Pt electrodes are susceptible to a severe transformation in which Pt oxide particles are formed and detached from the surface. In a cell configuration where the electrode is in direct contact with the polymeric membrane, these particles can irreversibly damage the membrane, affecting significantly production efficiency and severely limits the lifetime of the membrane. Boron-doped diamond (BDD) anodes have intrinsically a much higher solvent limit (oxygen evolution) so that the great majority of the current is actually used for the generation of ozone. Moreover, compared to other coated electrodes, UNCD®-coated Nb electrodes (Advanced Diamond Technologies Co., Romeoville, Ill.) are particularly stable and allow for a much higher current density without the physical damage observed on Pt electrodes. Ozone generation apparatus employing UNCD® electrodes (Advanced Diamond Technologies Co., Romeoville, Ill.) that are in contact with a polymeric membrane last much longer and maintain a higher current efficiency.
Nevertheless, conventional methods utilize a single pass mechanism/configuration in which water is flowed through the cell, reacted, and ejected. Through this process, ozone gas is produced and the water is ejected from the system. This results in a constant ozone output while water is flowed through the system, but results in low concentrations, which cannot be efficiently utilized and requires significantly large ozone generators in order to compensate for the lower efficiency. As such, the large size of the ozone generators and the inability of the conventional methods to provide a high concentration of readily available dissolved ozone have been limitations to widespread adoption of such technology.
Thus, there remains a need for a method of production of ozone from which results in smaller size ozone generators that can produce higher concentrations of readily available dissolved ozone.